Beneficiation of phosphate ores



Dec. 28, 1965 J. E. LAwvER BENEFICIATION OF PHOSPHATE CRES Filed Sept. 22, 1961 Pzosp/'ae Ore fj United States Patent O 3,225,923 BENEFIClATIN F PHOSPHATE ORES .lames E. Lawver, Lakeland, Fla., assgnor to International Minerals 8L Chemical Corporation, a corporation of New York Filed Sept. Z2, I1961, Ser. No. 140,048 9 Claims. (Cl. 209-3) This application is a continuation-in-part of application Serial No. 83,749, led lanuary 19, 1961, now abandoned.

This invention generally relates to the beneliciation of phosphate ores. More particularly, the invention relates to an electrostatic process for the concentration of phosphate ores pursuant to which a substantial portion of the phosphate values of the ore are recovered.

Phosphate ores comprising apatite or fluorapatite minerals normally occur in association with silica as a major gangue constituent. The phosphate industry has devoted substantial eifort to devising eflicient and economical methods for the production of a phosphate rock concentrate having a low silica content, preferably not more than about 2% to 4% by weight.

Separation of various materials including ores by electrostatic procedures long has been viewed by the art as a desirable expedient both from the standpoint of eiiiciency and economy. In practice, some success has been achieved in separating mixtures of conductive materials and mixtures of conductive and non-conductive materials. Electrostatic separation processes have also been developed for effecting separations between essentially nonconductive materials. One method of electrostatically beneficiating phosphate ores is described in Lawver U.S. Patent No. 2,805,769 issued September l0, 1957. Using the electrostatic method described in the Lawver patent, it is possible to substantially separate the phosphate minerals from the silica in phosphate ores.

In an electrostatic separation process it is preferred that the feed material be substantially dry. Phosphate ores are, however, usually conveyed from the mine site to the refinery as aqueous slurries, and wet screening methods are usually employed to separate the ore into various size fractions. When the phosphate ore is to be beneficiated in an electrostatic process, it is, therefore, necessary to dry the ore. The cost of drying the phosphate feed is, however, generally high, which contributes to the cost of the electrostatic beneiiciation process.

It is an object of the present invention to provide a novel process for the beneiiciation of phosphate ores.

It is another object of the present invention to provide a process for beneficiating phosphate ores wherein concentrates of commercial grade may be obtained at a high percentage recovery of BPL (bone phosphate of lime) values.

A further object of the present invention is to provide a process utilizing an electrostatic separation step for beneciating phosphate ores containing silica as a major gangue constituent.

These and other objects and advantages of the invention will be apparent to those skilled in the art from the description of the invention.

In accordance with the present invention, it has been discovered that eminently satisfactory beneficiation of phosphate ores and minerals can be achieved by means of a series of critical and interdependent process steps.

Generally described, the present invention is a process for beneiiciating a phosphate ore fraction comprising coarse and tine phosphate particles and coarse and ne silica particles, silica being a major gangue constituent of said ore fraction, which comprises removing a substantial amount of the relatively smaller size silica particles from said ore fraction to produce a phosphate fraction contain- 3,225,923 Patented Dec. 28, 1965 "ice ing a substantially lower amount of relatively smaller size silica particles, heating said fraction, inducing said fraction to accept differential electrical charges, and subjecting the charged fraction to an electrostatic separation operation.

The present invention is in part based on the discovery that an ecient separation of silica from the phosphate minerals in a phosphate ore may be effected electrostatically when a substantial amount of the relatively smaller size silica particles (ne silica particles) is removed prior to the electrostatic separation operation. It has been determined that the presence of smaller size silica particles is generally detrimental to the electrostatic separation. In general, it is preferred that a substantial amount of the silica particles having a size of -35 mesh be removed from the ore prior to the electrostatic separation. When the finer sized silica particles are removed, the remaining fraction which contains the relatively large size silica particles and small and large size phosphate particles may be very eiiiciently beneficiated by electrostatic means.

The ner or relatively smaller size silica particles are preferably removed in a cationic flotation operation which very efficiently floats off the finer silica particles. While the cationic otation operation is effective to float olf the liner silica particles, some of the relatively smaller size phosphate particles (tine phosphate particles) also tend to float off with the ner silica particles, which would lower the over-all recovery of the phosphate values if these finer phosphate particles were not recovered. Accordingly, in a preferred embodiment of the invention, prior to removing a substantial amount of the relatively smaller size silica particles, a substantial amount of the relatively smaller size phosphate particles, preferably the mesh particles, is removed from the ore fraction in an anionic otation operation using an amount of anionic dotation reagent less than that required to float substantially all of the phosphate particles in the ore fraction. It has now been determined that the smaller size phosphate particles may be eiiiciently removed in an anionic iotation effected with an amount of anionic flotation reagent less than that required to float substantially all of the phosphate content. The removal of a substantial portion of the finer or smaller size phosphate particles in the anionic ilotation operation materially benets a subsequent cationic iiotation to remove liner silica particles and the subsequent electrostatic separation steps. Therefore, in general, the effectiveness of the process of this invention is predicated on the combined and cooperative result of the successive operations, that is, in the correlation of the various types and sizes of particles removed or segregated in each step or stage of the process.

The invention is generally applicable to phosphate ores containing a phosphate mineral and silica as a major gangue constituent. Specic ores contemplated include Florida pebble phosphate, the various Tennessee phosphates, hard rock phosphates indigenous to the western United States and the various foreign phosphate ores such as Moroccan phosphates. The phosphate ore must, of course, be subdivided to a degree requisite to liberate the phosphate minerals from the silica. The phosphate ore is, therefore, reduced to economical liberation size to produce a granular material.

Florida pebble phosphate ore is usually delivered from the mine site to the beneiiciation plant as an aqueous slurry or pulp through pipe lines. In the washer section of the beneficiation plant l'og washers, water sprays, trommels, and sizing operations liberate the phosphate minerals from the silica or quartz and clays. In general, low grade Florida pebble phosphate, Tennessee phosphates, western phosphates, and other phosphate deposits are preferably comminuted by grinding to effect liberation of the phosphate values from the gangue and to effect reduction to a size where the material is amenable to beneficiation by electrostatic separation methods.

The liberated phosphate ore is preferably sized to produce phosphate ore fraction of a particle size less than about 4 mesh, preferably a fraction consisting of -8 mesh +200 mesh particles, and still more preferably a fraction consisting of -14 mesh +200 mesh particles. When phosphate ore fractions of the mesh size indicated above have the phosphate values substantially completely liberated from the silica and other gangue materials, the ore is rea-dy for further treatment in accordance with this invention.

It is of significance to note at this time that when the Florida pebble phosphate ore is of high grade, the +14 mesh pebble concentrate produced in the preliminary sizing is usually of suliiciently high grade so that it need not be further processed but may be shipped as a product concentrate.

In accordance with the present invention the liberated phosphate ore fraction is preferably subjected to a cat ionic flotation operation to remove a substantial amount of the finer silica from the ore.

It is recognized that the beneficiation of phosphate ores by froth iiotation has long been a common expedient widely practiced in the phosphate industry. As practiced, the liberated phosphate ore is usually first given a preliminary sizing to provide a +1 mm. fraction and a -14 mesh (about -l mm.) fraction. In turn the +14 mesh fraction is conventionally deslimed by removing substantial amounts of -200 material, which includes the clays or slime material. Frequently the desliming is effected to remove 150 mesh material. After desliming, the deslimed -14 mesh fraction is conventionally sized by means of classifiers, screens, trommels, hydroseparators .and the like to provide a -14+35 mesh fraction and a 35+150 or 200 mesh fraction. The c-oarser -14+35 mesh fraction, which is gener-ally considered to be too coarse for froth flotation, is beneficiated by means of spirals, shaking tables, and the like. The finer -35 mesh fraction is subjected to froth flotation.

A procedure known to the early art of concentrating phosphate ores embraced reagentizing a liberated phosphate ore with a cationic reagent effective selectively to coat a portion of the surfaces of the silica particles, followed by froth otation to produce a silica tail as a froth product and a phosphate concentrate as a depressed or sink product. Such a procedure, however, failed to afford a concentrate of satisfactory grade at high recovery. When the cationic froth flotation operation was conducted to float off large amounts of silica so as to produce a concentrate of commercially acceptable BPL content, many of the phosphate particles were carried over with the silica froth and were lost in the process, which gave a poor over-all recovery of the BPL values of the ore.

The phosphate industry, accordingly resorted to a combined process pursuant to which the liberated phosphate ores were subjected to both an anionic flotation and a cationic flotation in an effort to achieve satisfactory recovery of the phosphate values.

The development which lead to a first anionic flotation followed by a cationic flotation is described in detail in Crago U.S. Patent No. 2,293,640, Duke U.S. Patent No. 2,461,813, Duke U.S. Patent No. 2,661,842, and Hunter U.S. Patent No. 2,750,036. Such flotation processes refiect the general recognition by the art that only the more finely divided, e.g., 35 mesh, is well suited for flotation.

A second line of development for the fiotation of phosphate ores, wherein the phosphate ore is subjected to a first cationic otation and then to a subsequent anionic flotation, is presented in Tartaron U.S. Patent No. 2,222,728, Greene U.S. Patent No. 2,288,237, and Hollingsworth U.S. Patent No. 2,815,859. Again the processes were practiced on a rather finely divided ore, generally --0 mesh or finer.

Froth flotation processes, such as the above, must rely -on an accurate sizing of the liberated ore to .achieve eicient separation. In practice, however, it is extremely difficult to achieve an accurate sizing of the ore. Much of the -14 mesh liberated ore is characterized by a size of about 35 mesh. Further, Variation in the shape and specific gravity of the particles and their general tendency to blind screens further contribute to an inaccurate sizing and ultimately to a lower recovery of the phosphate values of the ore. Typical screen sizing of .a liberated phos phate ore are as follows:

Mesh Size Coarse Frac- Fine Fraction,

tion, Percent Percent It will be apparent that such processing, requiring extensive sizing equipment, does not provide an efficient method of concentration.

The process of the present invention, therefore, has a great advantage over the prior art processes in that the liberated phosphate ore need not be sized as is necessary in the prior art processes to provide two fractions, for example, a \-l4+35 mesh fraction which is beneciated by means of spirals, shaking tables and the like and a 35+l5-0 or 200 mesh fraction which is subjected to flotation. The cationic flotation step of the present invention is, therefore, also significantly different in principle from the prior art processes in that the cationic flotation feed of the present invention contains a substantial amount of +35 mesh particles, and even of :+20 mesh particles.

The liberated phosphate feed to the cationic flotation operation of the present invention, of course, contains relatively coarse or large size phosphate particles, fine or small size phosphate particles, coarse or large size silica particles and fine or small size silica particles. In a -14 +200 mesh phosphate ore fraction, the silica is present as particles of sizes substantially throughout this range and the phosphate mineral is also present as particles of sizes substantially throughout this range. In other words taking an arbitrary mesh size within these limits, e.g., 35 mesh, silica will be present as +35 mesh particles and as +35 mesh particles, and, similarly phosphate minerals will be present as +35 mesh particles and '-35 mesh particles.

As hereinbefore set forth, it has been determined that the smaller silica particles are particularly troublesome in an electrostatic separation operation and in accordance with the present invention a substantial amount of the smaller silica particles are removed as a preliminary preparation step for the electrostatic beneliciation. Predominantly silica fines are removed from the ore in the cationic flotation and the electrostatic separation process effectively and efiiciently separates the relatively coarser silica particles from the relatively finer and coarser phosphate particles.

The phosphate ore to be treated in accordance with the process of this invention, of a desired mesh size generally of at least about -10, preferably about 12, andi more preferably of -14 mesh, is Washed to remove slimes; and provide a deslimed ore fraction characterized by a maximum particle size or mesh size of about -10 or -121 or -14 mesh and a minimum particle size or mesh size of about 1+15O or 200 mesh. Such fractions are considered by the flotation art to be unsized fractions. Since the process of this invention efficiently processes such coarse ore fractions, the practice of this invention eliminates the need for extensive sizing and/or grinding to provide a very fine particle size.

The deslimed ore then is reagentized employing any suitable reagentizing procedure and any suitable cationic or positive ion flotation agent or any other reagent that will preferentially float silica. Many of such reagentizing procedures and reagents are known to the art. The cationic reagent is selected and used in amounts selected in accordance with principles will known to the art. Since only the finer silica needs to be floated, the amount of reagent used is preferably in the lower part of the ranges known to the art. In other words, it is preferred to use an amount of reagent less than that required to float off substantially all of the silica. Suitable cationic reagents include, inter alia, the higher aliphatic amines and their salts with Water-soluble acids; the esters of amino alcohols with high molecular Weight fatty acids and their salts with water-soluble acids; the higher alkyl-O-substituted isoureas and their salts with water-soluble acids; the higher aliphatic quaternary ammonium bases and their salts with water-soluble acids; the reaction product of polyalkylene polyamines with fatty acids and/ or fatty acid triglycerides; the higher alkyl pyridinium water-soluble acids; the higher alkyl quinolinium salts of watersoluble acids; and the like.

The reagentized ore is subjected to froth flotation employing any of the flotation equipment known to the art. The flotation is effective to remove, in the overflow, a substantial portion of the finer silica particles of the ore. The flotation is preferably conducted to remove at least 50% of the -35 mesh silica particles, and more preferably at least 75% of the -35 mesh silica particles in the feed to the flotation unit.

As hereinbefore set forth, while the cationic flotation operation is effective to float off the finer silica partcles, some of the finer phosphate particles also tend to float off with the finer silica particles which would lower the over-all recovery of the phosphate values if these finer phosphate particles were not recovered. In accordance with the preferred embodiment of the present invention, prior to the cationic flotation operation, the deslimed phosphate ore fraction is subjected to an anionic flotation operation to float off a substantial amount of the relatively smaller size phosphate particles. The anionic flotation is preferably conducted to remove at least 35% and more preferably at least 50% of the -100 mesh phosphate particles in the feed to the anionic flotation unit.

Therefore, in :accordance with this preferred embodiment of the invention, the deslimed ore fraction is reagentized employing anionic or negative ion flotation collectors and reagents selected in accordance with principles well known to the art. Instead of using a conventional amount of the anionic flotation reagent, the anionic flotation is effected with van effective amount of anionic flotation reagent substantially less than that required to float su'bstantially all of the phosphate content. The amount of anionic flotation reagent used is preferably less than about one-half that required to float substantially all of the phosphate. In a usual commercial flotation operation in which substantially all of the phosphate isfloated, from about 2.0 to about 5.0 pounds of anionic collector per ton of ore feed is used. Amounts of reagent less than that required to float substantially all of the phosphate particles may be denominated starvation quantities. In general, amounts of from about 0.01 to about 1.0 pound of flotation collector per ton of ore fraction is used in the starvation anionic flotation of this invention. Since the reagent when added to an aqueous pulp of the ore in the usual conditioning `or reagentizing step is attracted to the particles of high surface area per unit weight, the anionic flotation reagent is preferentially attracted to the finer phosphate particles and slimes or clays which were not removed in the desliming operation. Therefore, in the anionic flotation operation utilizing a starvation amount of anionic flotation reagent, since only the slimes or clays and finer phosphate particles have t-he flotation reagent thereon, the overflow from the flotation cell will consist predominantly of the slimes or clays and flner phosphate particles. The overflow from such an anionic flotation is generally of such high BPL content that it may be recovered as a product of the process but it also may be further treated as is hereinafter described in detail. The underflow from the anionic flotation operation is, accordingly, more slime free than the feed, and since only a starvation amount of anionic flotation reagent was used, substantially all of which is removed in the overflow, the underflow from the cationic flotation operation contains only very minor amounts of the anionic flotation reagent. Therefore, it is not necessary to acid scrub and wash the underflow from the anionic flotation unit before subjecting it to cationic flotation. The underflow may, therefore, be directly reagentized with cationic flotation reagent and subjected to cationic flotation to remo-ve the finer silica. As hereinbefore set forth, the anionic flotation operation is preferably effected to remove at least 35% and more preferably `at least 50% of the 100 mesh phosphate particles, and still more preferably at least of the -100 mesh phosphate particles in the feed to the anionic flotation operation.

The underflow from the cationic flotation unit is in accordance with the present invention subjected to an electrostatic separation operation.

In all electrostatic separations, it is necessary for satisfactory results that the surfaces of the particles be substantially dry, thus removing the complicating effects of moisture in the charging and separation steps. Additionally, it has been found that the ability of some mineral substances to assume and retain differential charges is materially affected, inter alia, by temperature. In the process of the present invention, separations are materially improved by conditioning the comminuted material at a temperature above that necessary merely to obtain dryness. Temperatures above 70 F. and preferably of from about F. to 350 F. are employed although higher temperatures below that at which the particular ore or mixtures undergoes incipient vitrification may be employed, i.e., temperatures of 900 F. or above. If desired, the heat-conditioned material may be cooled or allowed to cool before differential charging. However, cooling is not necessary and may not be desirable with some phosphate materials.

In accordance with the present invention, the dried particles are induced toy accept differential charges by the mechanism of contact electritication charging by contact electrication normally is obtained by one of two mechanisms, viz., contact with a metal or grounded metal surface or by particle-to-particle contact. When the difference in magnitude is great enough, effective separations are possible. When charging is effected by particle-to-particle contact under conditions present in the method of this invention, the phosphate particles and silica particles are charged to opposite pola-rity and separation is readily effected. While a relatively small amount of the charging obtained in the process of the invention results from particle-to-metal equipment contact, most of the charging is effected by the more desirable particleto-particle contact-either as a result of intentional agitation or agitation incident to handling of the feed material during and after the heating. The sign of the surface charge to be expected in particle-to-particle contact electrification depends `on the probability of the particle making contact with surface A, B, C, etc., and the relation of the surface energies that control the sign of contact electrification of the particles against A, B, C, etc.

It has been discovered that greatly improved differential charging of the particles is accomplished by essentially particle-to-particle contract while the dry comminuted material is maintained at a temperature of at least 70 F. Ideally, the particles would not contact a metal or grounded metal surface during the charging operation, since as previously indicated, contact with grounded metal surfa-ces usually causes all particles to accept a negative charge. On the other lhand, where the charging of the particles is accomplished essentially by particle-to-particle contact while at a temperature of at least 70 F., the surface charge found on the mineral species in the ore is equal and opposite in sign. Accordingly, the charge particles move in opposite directions in an electric field. Thus, in the process of the invention, it becomes possible effectively to separate nonconductive particles.

The desired particle-to-particle charging maybe effected in numerous ways, such as by tumbling the particles in a revolving drum or down an elongated chute in such quantity that contact between the particles and chute is at .a minimum. Alternatively, the particulate mixture, while maintained at the proper temperature, may be delivered from the drying apparatus to the electrostatic separator by means of a vibrating trough or any other suitable conventional means. At higher throughput, the great preponderance of charging is engendered by particle-to-particle contact rather than by contact of the particles with the apparatus. Suitable charging also may be obaincd by air agitation of the hot, comminuted mineral.

Following the preliminary heating and differential charging procedures the particulate mixture then is passed into a suitable electric field for separation. At the time of entry into the electrostatic field, the treated material should be at a temperature of at least about 70 F. and preferably at a temperature `of 140 F.350 F., although a higher temperature may be employed as previously indicated. Because of the nature of the treatment and charging of the material, the phosphate mineral particles and the silica gangue particles are strongly charged to opposite polarity with the result that a good separation often may be achieved in a single pass through an electrostatic field.

As long as the process limitations hereinbefore delineated are observed, the type of separation apparatus employed is not critical, the only limiting factor in terms of apparatus being that the charges on the differentially charged particles be substantially unaltered during delivery to and passage through the electrostatic field. Accordingly, apparatus wit-h which a material amount of charging by inductive conduction is obtained is not desirably employed. Further, apparatus using corona discharge should not 'be employed.

The so-called free-fall type of separator is preferred, inter alia, (a) because the usually employed elongated, vertically disposed electrodes provide longer residence times in the field, and v(b) because the apparatus is less expensive and more easily serviced. However, excellent separations may be achieved with roll-type apparatus wherein the conveyor roll is employed merely as `a means -of delivering the differentially charged particles to the electrostatic field and substantial charging by inductive conduction is avoided. Suitable free-fall type apparatus is disclosed in U.S. Patent No. 2,782,923 to Charles C. Cook et al. Suitable roll-type apparatus is disclosed in Taggart, Handbook of `Mineral Dressing, 1956, Chapter 13, such roll-type apparatus being 4operated to prevent substantial charging by inductive conduction.

The strength of the electrostatic field which will effectively alter the normal trajectory of ore particles depends on the mass of the particle and the total surface charge on the particle. The potential gradient desirably will vary from about 1,000 volts to about 5,000 Volts per inch of distance between electrodes in separating material of relatively fine particle size, and from about 3,000 volts to about 15,000 volts per inch for beneficiating coarser particles. In all such discussion of field strength, it must be borne in mind that corona discharges which ionize air are to be avoided. With free-'fall apparatus, it generally is preferred to operate with a total impressed difference of potential in the range of `about 30,000 volts to about 250,000 volts, While with a rolltype separator, a range of about 10,000 volts to about 8 715,000 volts is preferred. This voltage difference should be -maintained by means of a direct current potential source substantially free of ripple. A steady supply of direct current may be obtained with inexpensive filtering apparatus by the use of such equipment as a rectified radio frequency power supply.

Where ore particles are subjected to a series of separations, the feed to subsequent stages often will exhibit progressively reduced response to the electrostatic fields. This reduced response often may be due to loss or leakage of charges from the granular particles or coating lof the charged granular particles with fines. Such weakresponding concentrates may be restored or reactivated by passage through an impactor to create new surfaces and again recharging by frictional or other methods that give rise to differential electrification, which recharging may include a reheating in 'accordance with the treatment hereinabove described. Where a plurality of separation stages is employed it is also within the scope of the invention to water wash and redry a portion of the ore between stages.

When a satisfactory beneticiation is not accomplished in a single stage of electrostatic separation, the usual procedure is to `separate -a concentrate in a first or so-called rougher stage and to upgrade the Iconcentrate by treatment in two or more so-called cleaner electrostatic separation stages. While the breadth of t-he range of temperature at the times of passage through the electrostatic field appears adequate t-o allow for cooling during passage through a plurality of stages, it frequently happens that this is not the case. One of the primary reasons for this failure is that in order to reduce the number of separation stages, it is desirable to make the first or rougher separation at or near the temperature of about 175 F. to about 300 F., depending on the character of the ore. Concentrate from the rougher separation thus will cool more or less rapidly depending upon the difference in temperature between 200 F. and the atmospheric temperature. After passage through one or more concentrate upgrading stages, it is found that sometimes the phosphate material has cooled below a temperature at which a measurable degree of upgrading will occur. When the phosphate ore has become 4too cool, or picked up too much surface moisture, it sometimes happens that neither the concentrate nor the tail product will respond to further passes through electrostatic fields of the same, lower, or higher potential gradient.

In general, it has been found that where additional passes through an electrostatic field are desirable, a secondary heat treatment whereby the temperature of the solids is maintained at or is raised to or above 200 F. between separation stages following the first or rougher separation not only produces concentrates of higher BPL content but also reduces the number of separation stages to obtain concentrates of relatively high BPL content. Optimum conditions, of course, exist for various ores and for the various types of electrostatic separators and readily may be determined.

The method of this invention is illustrated in the flow sheet in the accompanying drawing. The drawing represents a diagrammatic flow sheet of one preferred method of this invention.

Referring to the drawing, a phosphate ore 1 is subjected to a sizing operation in sizing zone 3 to produce a +14 mesh fraction which is generally of sufficiently high BPL content to be recovered as product. The -14 mesh material is deslimed in desliming zone 5. The desliming is illustrated as being on --200 mesh; however, as hereinbefore set forth, the desliming may be on any other suitable mesh size readily selected in accordance with principles well known in the art. The -14-|20O mesh fraction 7 recovered from the desliming zone may be directly passed to the cationic flotation unit 13 as is illustrated by dotted line 9; however, in accordance with one embodiment of the present invention the deslimed phosphate ore fraction 7 is mixed with an anionic flotation reagent composition and delivered to anionic flotation unit 11. A relatively small amount of flotation reagent is employed so as to remove substantially only the smaller phosphate mineral particles in accordance with the principles hereinbefore set forth. The overflow from the anionic lflotation unit 11 will be composed primarily of fine phosphate particles and clays and may be recovered as a product of the process or, as illustrated in the drawing, it may be introduced into dewatering zone for further processing. The underflow from the anionic flotation unit, without an acid scrubbing or washing operation, is mixed with a cationic flotation reagent composition and delivered to cationic flotation unit 13. The overflow from the flotation unit 13 will be composed primarily of silica and heavy minerals. The underflow from flotation unit 13 has substantially less fine silica, which was removed as overflow. The underflow is dewatered at a dewatering zone 15 and heated in heating zone 17 which may take the form of a vertical type heat exchanger, a rotary kiln, a tunnel kiln, a multiple hearth furnace or other suitable heating devices. The ore while still at an elevated temperature is passed into a charging unit 19. The charged ore is then introduced as free falling bodies into a suitable electrostatic separator 21.

As a result of the passage of the material through the electrostatic field, a phosphatic concentrate 23 is separated which is a saleable product. The electrostatic concentrate 23 has a higher BPL value than the feed to the electrostatic unit since a substantial amount of the coarser silica is separated therefrom as atail fraction. The tail fraction 2S may be of sufficiently high BPL value to be used as a low grade phosphate concentrate.

In an alternate procedure as shown by the dotted lines in the drawing, the tail fraction 25 is delivered to an impactor 27, for example, a hammer mill, to expose clean surfaces on the particles. The impacted material from the impactor is preferably recycled to the flotation unit 13, or to the heating unit 27, or to the anionic reagentizing step for anionic flotation in flotation unit 11, or to some other suitable point in the process.

In order to give a fuller understanding of the invention, but with no intention to be limited thereto, the following specific examples are given.

EXAMPLE I A deslimed Florida pebble phosphate ore fraction (-14-l-200 mesh) was subjected to froth flotation employing 0.30 lb./ton. of an amine flotation reagent and 0.4 lb./ton of kerosene extender. No caustic was used.

The feed, concentrate, and tail analyzed as follows:

The data in the above table illustrates that only a small amount (6.2%) of the BPL values in the feed were lost as overflow in the cationic flotation while still removing 52.8% of the weight of the charge and over 50% of the -35 mesh silica in the charge. The concentrate which is electrostatically beneficiated in accordance with this invention is only 47.2% by weight of the feed and contains 93.8% of the BPL values of the feed.

The 47.2% may, of course, because of the lesser weight be more economically dried than the deslimed phosphate ore charged to the flotation operation.

The phosphate concentrate from the cationic flotation was then dried and heated to 350 F., cooled to a temperature of 220 F., and differentially charged by parl0 ticle movement in the feeder. The differentially charged ore, at a temperature of about 200 F. was then dropped as freely falling bodies through a free-fall type electrostatic separator. The field was maintained between the spaced vertical electrodes of the separator at a gradient of approximately 10,000 volts per inch.

The results of the single pass separation were as follows: A

A deslimed Florida pebble phosphate ore fraction (-14-l-200 mesh) of` about 34.5% BPL content was subjected to froth flotation employing 0.20 lb./ton of an amine flotation reagent and 0.4 lb./ton of kerosene extender. No, caustic was used.

The concentrate had a 62.16% BPL content and was 48% by weight of the feed and contained 98.19% of the BPL values in the feed. The flotation tailing was 52% by weight of the feed and contained more than 50% of the -35 mesh silica in the feed. Therefore only 1.81% of the BPL values in the feed were lost as overflow in the cationic flotation while still removing 52% of the Weight of the charge. The concentrate which is electrostatically beneciated in acordance with this invention is only 48% by weight of the charge. The 48% may, of course, because of the lesser weight, be more economically dried than the deslimed phosphate ore feed charged to the flo tation operation. i

rl`he phosphate concentrate from the flotation operation was then dried, differentially charged and subjected to an electrostatic separation substantially as described in Example I. The electrostatic concentrate recovered had a BPL content of 75.1%. BPL and was 37.5% by weight of the flotation feed and contained 92.6% of the BPL values in the feed to the flotation operation; in otherl words, the overall recovery of BPL values was 92.6% by weight.

EXAMPLE III This example illustrates the embodiment of the present invention wherein the deslimed unsized ore fraction is sub jected to an anionic flotation prior to the cationic flotation for the removal of silica.

A deslimed Florida pebble phosphate ore fraction (-14-l-200 mesh) of about 23% BPL content was subjected to anionic flotation employing 0.1 lb./ton of fatty acid flotation collector and 0.2 lb./ton of kerosene extender. The float concentrate from this anionic flotation operation had a 70.03% BPL content and represented 8.36% by weight of the feed and it also represented 25.09% of the BPL values in the feed to the anionic flotation unit.

The underflow from the anionic flotation unit, without acid scrubbing or washing, was then subjected to cationic froth flotation employing 0.20 lb./ton of an amine flotation reagent and 0.4 lb./ ton of kerosene extender. No caustic was used. The overflow or silica tailing from the cationic flotation unit represented 54.28% by weight of the original feed to the anionic flotation unit. The silica tailing had only a 0.82% BPL content and represented only 1.91% of the BPL values in the original feed to the anionic flotation unit.

The underflow from the cationic flotation unit was dried, differentially charged and subjected to an electrostatic separation substantially as described in Example I. The electrostatic concentrate recovered had a BPL content of 71.08% BPL and was 23.39% by Weight of the flotation feed and contained 71.27% of the BPL values in the feed to the anionic flotation operation. The tailing from the electrostatic operation was 13.97% by Weight of the anionic flotation feed and had a 2.57% BPL content which represented 1.73% of the BPL values in the original feed to the anionic flotation.

By combining the float concentrate (70.03% BPL and 25.09% recovery) with the electrostatic concentrate (71.08% BPL and 71.27% recovery), a combined product of 70.80% BPL is obtained and the combined product represents a 96.36% (25.09-|-71.27) overall recovery of BPL values.

The above examples illustrate that a high recovery of phosphate values may be achieved in accordance with the present invention. The process is economical and has advantages over prior processes as has been pointed out in the specification. v

The description of the invention utilized specific reference to certain process details; however, it is to be understood that such details are illustrative only and not by way of limitation. Other modifications and equivalents of the invention will be apparent to those skilled in the art from the foregoing description.

Having now fully described and illustrated the invention, what is desired to be secured and claimed by Letters Patent is set forth in the appended claims.

I claim:

1. A process for beneficiating a substantially deslimed, -4-i-200 mesh Florida pebble phosphate ore fraction comprising coarse and ne phosphate particles and coarse and line silica particles, silica being a major gangue constituent of said ore fraction which comprises removing a substantial amount of the relatively smaller size silica particles from said ore fraction as a oat product in a cationic froth flotation operation to produce an intermediate phosphate ore fraction containing a substantially lower concentration of relatively smaller size silica particles as compared to the pebble phosphate ore fraction feed, heating said intermediate phosphate ore fraction to a temperature of at least 70 F. to dry said fraction, differentially charging said dried fraction, passing the differentially charged fraction through an electrostatic field at a temperature not greater than about 500 F., and recovering a phosphate concentrate.

2. A process according to claim 1 in which the smaller size silica particles are 35 mesh and are removed in said cationic froth flotation.

3. A process according to claim 1 in which said Florida pebble phosphate ore fraction is -14-1-200 mesh.

4. A process according to claim 1 in which said Florida pebble phosphate ore fraction is -8-1-200 mesh.

5. A process for beneliciating a substantially deslimed -4-i-200 mesh phosphate ore fraction containing |35 mesh and -35 mesh size phosphate minerals and +35 and -35 mesh size silica, said silica being a major gangue constituent of said ore which comprises removing a substantial amount of said -35 mesh silica from said ore fraction as a float product in a cationic froth otation operation to produce an enriched phosphate fraction containing a substantially lower amount of -35 mesh silica as compared to the phosphate ore fraction feed, heating said enriched phosphate fraction to a temperature of at least 150 F., inducing said heated fraction to accept differential electrical charges, and subjecting the charged fraction to an electrostatic separation operation.

6. A process according to claim 5 in which at least of the -35 mesh silica in the phosphate ore fraction is removed in said cationic froth otation.

7. A process according to claim 5 in which at least 75% of the -35 mesh silica in the phosphate ore fraction is removed in said cationic froth flotation.

8. A process according to claim S in which the differential charging of said enriched phosphate fraction is by particle-to-particle contact electrication.

9. A process according to claim 5 in which said phosphate ore fraction is a substantially deslimed -14-1-200 mesh Florida pebble phosphate fraction.

References Cited by the Examiner UNITED STATES PATENTS 2,744,625 5/1956 Houston 209-l2 2,805,769 9/1957 Lawver 209-127 2,967,615 1/1961 Goin 209-12 2,997,171 8/1961 Samsel 209-4 HARRY B. THORNTON, Primary Examiner.

FRANK W. LUTTER, ROBERT A. OLEARY,

Examiners. 

1. A PROCESS FOR BENEFICIATING A SUBSTANTIALLY DESLIMED, -4+200 MESH FLORIDA PEBBLE KPHOSPHATE ORE FRACTION COMPRISING COARSE AND FINE PHOSPHATE PARTICLES AND COARSE AND FINE SILICA PARTICLES, SILICA BEING A MAJOR GANGUE CONSTITUENT OF SAID ORE FRACTION WHICH COMPRISES REMOVING A SUBSTANTIAL AMOUNT OF THE RELATIVELY SMALLER SIZE SILICA PARTICLES FROM SAID ORE FRACTION AS A FLOAT PRODUCT IN A CATIONIC FROTH FLOTATION OPERATION TO PRODUCE AN INTERMEDIATE PHOSPHATE ORE FRACTION CONTAINING A SUBSTANTIALLY LOWER CONCENTRATION OF RELATIVELY SMALLER SIZE SILICA PARTICLES AS COMPARED TO THE PEBBLE PHOSPHATE ORE FRACTION FEED, HEATING SAID INTERMEDIATE PHOSPHATE ORE FRACTION TO A TEMPERATURE OF AT LEAST 70*F. TO DRY SAID FRACTION, DIFFERENTIALLY CHARGING SAID DRIED FRACTION, PASSING THE DIFFERENTIALLY CHARGED FRACTION THROUGH AN ELECTROSTATIC FIELD AT A TEMPERATURE NOT GREATER THAN ABOUT 500*F., AND RECOVERING A PHOSPHATE CONCENTRATE. 